A light guide as herein contemplated may be either a single optical fiber, a group of optical fibers arrayed in a flat bundle or ribbon, or a light-conducting foil. In each instance the light guide has internally reflecting boundaries with a critical angle of reflection determined by the difference between the refractive indices of the guide substance and the surrounding medium. As is well known, light rays striking the guide boundary at a glancing angle, not exceeding the critical value, are totally reflected and thus do not leave the confines of the guide. In traveling along their transmission path, they bounce back and forth between opposite guide surfaces and eventually leave the exit end of the guide at an inclination to its axis which depends upon the angle of incidence.
Theoretically, at least, a ray may pass along the axis of a straight guide without internal reflection. Such a ray has the shortest transit time through the guide in comparison with rays undergoing reflection, the longest time being that of a ray exiting from the guide surface at the critical or guidance angle. These relative delays of light rays originating at a common modulated source result at the receiving end in a broadening of the pulses and thus in a distortion of the signal. That distortion, of course, increases with the length of the transmission path.
A variety of equalizers have already been suggested for dealing with this problem. One such equalizer, described by D. C. Gloge in an article entitled "Fiber-Delay Equalization by Carrier Drift in the Detector", Optoelectronics, vol. 5, 1973, pages 345-350, operates electronically on the electric pulses derived from the luminous signal at the receiving end; the light rays emerging at different angles from the exit end of an optical fiber are electronically detected in separate zones working into delay lines which introduce compensatory differences in transit time. Such a system, requiring active electronic components, is relatively complex and limited to specific radiation receivers. In commonly owned U.S. Pat. No. 4,094,578 granted to me jointly with Riccardo Vannucci, an optical signal-transmission system has been disclosed and claimed in which the equalization of the light paths is carried out with the aid of mirrors interposed between cascaded light guides angularly adjoining one another.
Other solutions, such as those suggested in U.S. Pat. Nos. 3,759,590 and 3,832,030, provide optical equalizers with refractive cones or lenses serving for a compensatory refraction of light rays incident at different angles.
The presence of three or more refractive bodies between confronting guide ends in systems of the last-mentioned type results in a significant attenuation of the luminous radiation, especially for slanting light rays which strike the surfaces of these bodies at almost a glancing angle and are therefore subject to heavy Fresnel losses.
In my above-identified copending application Ser. No. 779,821 I have disclosed a path-length equalizer comprising two identical transparent lens members of positive refractivity spaced apart along a common axis or centerline of the aligned light guides between which the equalizer is disposed, each of these lens members having a cross-section in at least one longitudinal plane of symmetry of the light guides which consists of two symmetrical truncated lens profiles having a boundary on that centerline. That boundary is offset from the optical axes of the truncated lens profiles, these axes thus lying on opposite sides of the centerline; each lens profile extends from the centerline (and therefore also from the aforementioned boundary) to at least a point of interception of a limiting ray converging at the closer light-guide end, the path of such limiting ray extending from that point of interception at a lens profile of one lens member to the geometrical center of the other lens member and thence substantially along the centerline to the more distant light-guide end.
With an equalizer of this type I have been able to reduce the broadening of a signal pulse in such an optical transmission system by up to about 75%. While that amount of reduction is satisfactory in many instances, situations exist -- especially with long transmission paths -- where the remaining signal distortion is still inadmissible.
In my other copending application referred to above, Ser. No. 793,420, I have disclosed an equalizer in the form of a transparent body or block with a nonuniform index of refraction varying symmetrically from the centerline outwardly within the aforementioned longitudinal plane of symmetry, this body having a transverse entrance face and a transverse exit face spaced by the same distance s from the light-emitting end of one light guide and from the light-collecting end of the other light guide, respectively. The limiting light rays leaving the center of the emitting guide end at a critical angle .THETA..sub.M with reference to the centerline strike the entrance face at points spaced from that centerline by a distance r.sub.M =s.multidot.tan.THETA..sub.M, the refractive index increasing progressively from the centerline outwardly over substantially the distance r.sub.M and thereafter decreasing progressively over a further distance which preferably equals or exceeds r.sub.M and in any event must be sufficient to deflect the limiting ray within the body back toward the centerline over a generally sinusoidal first path ending substantially tangentially to the centerline at the exit face whereby that ray continues substantially along the centerline to the collecting guide end. On the other hand, thanks to the reciprocity of the ray paths, a central or paraxial ray leaving the emitting guide end substantially along the centerline is deflected within the body over a generally sinusoidal second path which is longitudinally shifted with reference to the first path and ends at the exit face substantially at a distance r.sub.M from the centerline whereby this central ray continues to the collecting guide end substantially at the critical angle .THETA..sub.M. Thus, the paraxial and limiting rays are effectively transposed to equalize their path lengths. If the variations of the refractive index of the transparent body on each side of the centerline within the aforementioned longitudinal plane (or within any such plane in the case of a centrally symmetrical guide system) represent a substantially symmetrical function of distance having an inversion point at the distance r.sub.M, the various ray paths within the body will be substantially symmetrical about an ancillary axis spaced from the centerline by the distance r.sub.M. In that case the length of the body between its two transverse faces should be substantially equal to 3/2 times the distance between successive points of intersection of any sinusoidal ray path with that ancillary axis, this length being then equal to three quarter-wavelengths of the sine curve represented or approximated by these paths.
The one-body equalizer just described yields results similar to those attainable with the two lens members of an equalizer according to my first-filed prior application. In both instances the path lengths of light rays exiting from the emitting guide end at near-critical angles and at near-zero angles are made substantially the same, yet no special consideration is given to the intermediate rays.